1. Field of the Invention
The present invention relates to an image forming apparatus and, more particularly, to an image forming apparatus such as a digital copying machine or a printer.
2. Description of the Related Art
FIG. 4 is a cross-sectional view showing a part of a fixing unit and an automatic duplex unit (ADU) of a conventional image forming apparatus. A printing paper PP onto which a toner image has been transferred from a photoconductor drum disposed in a toner image forming section is subjected to fixing processing performed by a heating roller 133 and a pressure roller 134 of the fixing unit 130 and fed to a fixing and discharging roller 135. The fixing and discharging roller 135 then feeds the received printing paper PP to a reverse gate 136.
In the case where a printing paper PP onto only one side of which a toner image has been transferred is reversed and discharged outside the machine, the reverse gate 136 feeds the printing paper PP received from the fixing and discharging roller 135 toward a paper re-feeding route 132a. In this case, the printing paper PP is fed through a feeding path that turns at an acute angle (in FIG. 4, turns at right angles) from the fixing and discharging roller 135 toward the paper re-feeding route 132a. The printing paper PP guided by the paper re-feeding route 132a is fed in a switchback manner by reverse feeding rollers 137a and 137b toward a printed paper discharge port 131.
In the case where duplex printing is performed, when a toner image has been transferred onto only one side of the printing paper PP, the reverse gate 136 feeds the printing paper PP received from the fixing and discharging roller 135 toward the paper re-feeding route 132a as in the above case. The printing paper PP guided by the paper re-feeding route 132a is fed in a switchback manner by the reverse feeding rollers 137a and 137b toward an automatic duplex unit 170. The automatic duplex unit 170 feeds the printing paper PP that has been fed thereto in a switchback manner toward the toner image forming section for paper re-feeding using reverse feeding rollers 137c and 137d. 
In the case where one-side printing or duplex printing for the printing paper PP has been completed as instructed and where the printing paper PP is to be discharged outside the machine without being reversed, the reverse gate 136 directly guides the printing paper PP that has been fed from the fixing and discharging roller 135 to the printed paper discharge port 131.
FIG. 5 schematically shows the state where the printing paper fed from the fixing and discharging roller 135 toward the paper re-feeding route 132a in an easy-to-understand manner. In the conventional machine, a rotational speed V3 of the reverse feeding roller 137 (137a, 137b, 137c, 137d) is set equal to a rotational speed V1 of the heating roller 133 and pressure roller 134. Only a rotational speed V2 of the fixing/discharge roller 135 is set higher than the V1 and V3. The reason that only the rotational speed V2 is set higher is to prevent image stain from being produced when the printing paper is scratched by a peeling claw disposed in the fixing unit (refer to, for example, Jpn. Pat. Appln. Laid-Open Publication No. 6-156852 (pages 3 to 6, FIG. 1) and Jpn. Pat. Appln. Laid-Open Publication No. 7-267454 (pages 4 to 7, FIG. 1)).
However, in the conventional image forming apparatus, the leading end of the printing paper fed toward the reverse gate 136 hits against the reverse gate 136 to generate early “swelling” (refer to FIG. 5). When the printing paper is passed through the reverse gate 136 and is further fed, this “swelling” moves to the rear side of the printing paper, with the result that the “swelling” remains at the rear side portion of the printing paper as “bending” or “curl”. In the case where the printing paper is reversed and re-fed to the toner image forming section, the “curl” stands at the leading end of the printing paper. In this case, when the printing paper has a bigger size and smaller thickness, the “curl” is likely to become greater.
Here, curl amount in an LD size printing paper was measured under the condition that V1 and V3 were set to 420 mm/sec, V2 was set to 448 mm/sec or 429 mm/sec, and rotational speed V3 of reverse feeding roller/rotational speed V2 of fixing and discharging roller was changed by ±4%. As a result, the data as shown in the following Table 1 was obtained. In this case, V2H denotes the case where V2 is 448 mm/sec, and V2L denotes the case where V2 is 429 mm/sec.
TABLE 1V3/V2 (%)V2H (mm)V2L (mm)−3.9——−3.7—16−3.0—13−2.23811−1.5229−0.71811+0.11010+1.0810+1.7510+4.059
FIG. 6 is a graph showing the measurement result corresponding to Table 1. As can be seen from Table 1 and graph of FIG. 6, the smaller the value of rotational speed V2 of the fixing and discharging roller, the smaller the curl amount becomes (when V3/V2 (%) becomes a positive value, curl amount is reduced). That is, the decrease in the rotational speed of V2 of the fixing and discharging roller reduces the curl amount, with the result that it is possible to keep the shape of the printed printing paper in an original state. On the other hand, the decrease in the rotational speed of V2 of the fixing and discharging roller may allow the printing paper to be scratched by the peeling claw disposed in the fixing unit, producing image stain. As another means that reduces the curl amount, special equipment such as a dedicated de-curling unit or a heat pipe has been utilized.